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3 AMP fuse keeps blowing out on Carrier furnace

11-18-2012, 12:41 AM

Hello, I have a Carrier furnace that is suddenly blowing the 3 amp fuse. I finally isolated that the COM terminal is blowing the fuse. If I connect the blue t-stat wire to it, the fuse will blow. I did this by disconnecting each terminal wire one at a time (W-R-G etc) I am pretty sure that having the t-stat (furnace)in the "off" position, it did not blow. I think it happened when the furnace was turned on at the t-stat. I did so much trouble shooting, I'm forgetting what I did. I already had the a/c wires disconnected. Is this a circuit board problem? Thanks for any help

If you disconnected the wire at the c terminal and then call for heat and fuse doesnt blow it sounds kind of like a t-stat issue. The com terminal on the pcb is often used to power condensers and can be used as primary power for some digital t-stats. I would check the stat and then make sure none of the low volt wires or switches are going to ground. Also if its a heat pump you might check for a bad contactor at outdoor unit.

Comment

I did actually try another t-stat. That's not the problem, sadly. Can I hook up the 2 a/c wires without using the Com terminal and ignore the terminal altogether? My t-stat is battery operated. Thanks for your help

Actually, COM is common ground. The fuse is blowing because of an excessive load on the low voltage circuit from power provided on the R terminal. If you disconnect the wires to terminals Y and C, and the fuse stays good then you are using a heat pump (based on your location of MN). Look for shorted contactor wires, a shorted contactor, or bad heat pump control board (if applicable). If the fuse still blows with Y and C disconnected then the short is elsewhere in the low voltage circuit.

Edit:To answer the other question... you *could* connect the ground to something other than C, however, as the load is still too much, the fuse will blow.

Comment

Hi, actually heat pumps are not very common here and there is not a heat pump invlved. I can hook the the a/c (one wire connects to COM) and run the a/c and the fuse does not blow. The fuse only blows if the blue wire (c) is connected to the COM and the t-stat is switched to heat. It's fine if switched to a/c. Thanks for your comments

Comment

Sounds like maybe your w is shorting to c. You might check the wiring for shorts or as a simple test try substituting the blue wire with a spare if there is one available. With a volt meter you can test for continuity between the low voltage wires. This you have to test with all wires d/c. Is this a new install or a problem that has suddenly appeared? Could be miswired.

if you take off the thermostat (off the base or wall plate) and the fuse stays good then the wiring is ok and the thermostat is bad

if the fuse still blows, the tstat is probably ok and the short is elsewhere

what terminal does the blue wire connect to in the thermostat, and what is the make and model of the thermostat?

not necessarily. The thermostat takes low voltage power from a wire coming from the furnace and when it's time to call for heat it connects this wire to another wire going back to the furnace to supply the furnace with a low voltage signal letting it know that there is a call for heat. If the wire carrying the signal back to the furnace is grounded out, you can disconnect the t-stat and it will never blow the fuse. If this wire is grounded out, it will also cause the furnace to only blow the fuse when there is a call for heat.